This invention relates to a key telephone system comprising a main device or a key service unit connected to an office line and a plurality of key telephone units.
With an increase in the amount of information communications in recent years a key telephone system combining the effective utilization and the function of an interphone has been used extensively.
A prior art key telephone system comprises a main device including relays and a source of power, a plurality of telephone units each provided with key buttons for selecting office lines, and an office cable for interconnecting the telephone units. To provide a response and a transmission to a particular office line, an office line selection key corresponding to the particular office line is manually depressed thereby enabling a single key telephone unit to utilize many office lines. Examples of such system are disclosed in Japanese patent publication No. 17,449 of 1976 and U.S. Pat. No. 3,115,551.
The office cable used in such a key telephone system generally comprises a pair of talking lines, a control line and a plurality of lines for interconnecting telephone sets by the manual operation of keys and the number of the lines comprising the cable increases with the number of telephone sets so that the wiring operation of the cable becomes troublesome and requires much labour.
In addition to the cost and labour required for the system the trouble caused by misconnections increases also. Further such system requires the use of wiring connectors of the multiple terminal type, and the diameter of the cable increases. Consequently, a large space is required to install the cable and the cost of the cable increases.
In the prior art key telephone system a plurality of switching networks corresponding to the plurality of telephone units are installed in the main device. Since such switching network is used to interconnect an office line corresponding to an office line selection key button and a telephone unit in accordance with an information or data corresponding to said key button, while a user of a telephone unit is talking through one office line, the content of the speech is tapped by another user if he hangs off the handset and then depresses an office line selection key corresponding to said office line. Accordingly, it is necessary to prevent such tapping.
Heretobefore such tapping has been prevented by adding a relay to the button telephone set, which renders operative or inoperative an office line between a switching network and a telephone line circuit of a given telephone unit when the office line is in a busy condition. With this method, however, it is necessary to provide such relay for each telephone unit thus complicating the system. Further, as the number of the telephone unit increases, the number of the lines of the cable interconnecting the main device and the telephone units increases so that the connecting operation of the cable becomes troublesome. This increases the cost of installation.
Moreover, in such secret telephone system it is impossible to release the secrecy. If possible, all telephone sets are released from secret state so that it is impossible to release the secret state of a particular telephone set.
Further, in the prior art key telephone system, when the source in the main device becomes inoperative due to interruption of the supply of electric power for example, the main device will be completely isolated from the office lines thus rendering inoperative the entire system. For this reason, at the time of the failure of the source it has been necessary to make it possible to talk by using a particular office line by closing a switch.